1. Field of Invention
The present invention relates to an exhaust passage switching unit and method for an internal combustion engine and, more particularly, to an exhaust passage switching unit and method capable of precisely judging the state of a switching mechanism for switching an exhaust passage.
2. Description of Related Art
Exhaust passage switching units as disclosed, for example, in Japanese Patent Application No. 8-334014 have been known. A switching unit of this type is equipped with a switching valve which switches exhaust gases to a main exhaust passage in which a catalyst is disposed or to a bypass passage. In an operating state with a high rotational speed and a high load, deterioration caused by overheating of the catalyst is prevented by switching the exhaust passage to the bypass passage.
When the main exhaust passage serves as the exhaust passage, the flow resistance of the exhaust passage increases because of the resistance of the catalyst. If the flow resistance of the exhaust passage increases, the negative pressure in an intake pipe decreases accordingly. Hence, according to the switching unit of the related art, it is determined, based on a change in negative pressure in the intake pipe when a switching signal is supplied to the switching valve, whether or or the exhaust passage has been switched, that is, whether or not the switching valve is in normal operation.
However, the negative pressure in the intake pipe changes depending not only on a flow resistance of the exhaust passage but also on an operating state of the internal combustion engine. Thus, it is difficult to distinguish between a change in negative pressure in the intake pipe caused by the switching of the exhaust passage and a change in negative pressure in the intake pipe caused by an operating state of the internal combustion engine. The method in the aforementioned switching unit of the related art does not necessarily permit precise judgment of an operational state of the switching valve.